This application claims the benefit of Japanese Application No. 2001-386953 filed Dec. 20, 2001.
The present invention relates to an MR imaging method and an MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method and an MRI apparatus that can suppress degradation in visibility of an image by a missing line due to the subject motion.
In MRI, an LSDI (line scan diffusion imaging) technique is known as an example of DWI (diffusion weighted imaging) pulse sequences for imaging microscopic motion of water molecules, etc.
One example of the LSDI technique is disclosed in U.S. Pat. No. 5,786,692, for example.
In the LSDI technique, the step of selectively exciting a linear region and applying an MPG (motion probing gradient) pulse to collect data is repeated in a slice plane, and an MR image is reconstructed using the collected data.
FIG. 9 shows an example of an MR image obtained by the imaging according to the LSDI technique.
The MR image G1 exhibits a region of lesion xcex1 in white that has a diffusion altered by infarction etc. within an imaged site (e.g., brain tissue) B.
FIG. 10 shows an example of an MR image when the subject moved during the imaging according to the LSDI technique.
In the MR image G2, a so-called xe2x80x9cmissing linexe2x80x9d F occurs, in which a portion of an image drops off to form a black line.
As described with reference to FIG. 10, if the subject moves during imaging, the missing line F occurs in an image, resulting in a problem of an image with degraded visibility.
It is therefore an object of the present invention is to provide an MR imaging method and an MRI apparatus that can suppress degradation in visibility of an image by a missing line due to the subject motion.
In accordance with a first aspect, the present invention provides an MR imaging method for reconstructing an MR image using a pulse sequence according to an LSDI technique in which the step of selectively exciting a linear region and applying an MPG pulse to collect data is repeated, said method characterized in comprising: detecting data affected by the subject motion; and re-imaging a region corresponding to said data.
In the MR imaging method of the first aspect, when a region corresponding to data affected by the subject motion is detected, the region is re-imaged; therefore, data with no (or reduced) effect of the subject motion can be collected. Thus, the number of missing lines is reduced, and an image with enhanced visibility can be displayed. This leads to no additional load to the subject because only a slight increase of imaging time is required for the re-imaging of a small region.
In accordance with a second aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that the re-imaging is terminated when the number of repetitions of the re-imaging becomes equal to or greater than a predetermined upper limit.
In the MR imaging method of the second aspect, since the number of repetitions of the re-imaging is limited, a disadvantage of endless re-imaging of the same region and reduction of imaging efficiency can be avoided.
In accordance with a third aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that the re-imaging is terminated when the re-imaging time becomes equal to or longer than a predetermined maximum time.
In the MR imaging method of the third aspect, since the re-imaging time is limited, a disadvantage of a too long re-imaging time can be avoided.
In accordance with a fourth aspect, the present invention provides an MR imaging method for reconstructing an MR image using a pulse sequence according to an LSDI technique in which the step of selectively exciting a linear region and applying an MPG pulse to collect data is repeated, said method characterized in comprising: detecting data affected by the subject motion; and employing interpolated data in place of said data.
In the MR imaging method of the fourth aspect, when data affected by the subject motion is detected, interpolated data is employed in place of the data; therefore, the number of missing lines can be reduced without lengthening the imaging time.
In accordance with a fifth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that the interpolation is made using a neighbor-weighted interpolation technique.
In the MR imaging method of the fifth aspect, the neighbor-weighted interpolation technique provides interpolation with good accuracy.
In accordance with a sixth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in further comprising: defining beforehand a line range to be detected; and detecting data affected by the subject motion only in said line range.
In the MR imaging method of the sixth aspect, since the effect of the subject motion is detected for a limited line range (usually, a line range in the center of an image) that highly warrants prevention of the missing line, the apparent image quality can be improved while minimizing through-put decrease by the re-imaging.
In accordance with a seventh aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in further comprising: determining beforehand a decision reference value for each site; and comparing the decision reference value corresponding to an imaged site with an evaluation value based on the collected data to detect data affected by the subject motion.
In the MR imaging method of the seventh aspect, since decision reference value is determined beforehand for each site, the detection of data affected by the subject motion can be achieved by a simple comparison calculation.
In accordance with an eighth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in further comprising: calculating a decision reference value based on the result of a prescan; and comparing said decision reference value with an evaluation value based on the collected data to detect data affected by the subject motion.
In the MR imaging method of the eighth aspect, since the decision reference value is calculated by a pre-scan, a decision reference value accommodating individual differences among subjects can be determined.
In accordance with a ninth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in further comprising: calculating a decision reference value based on the data being collected; and comparing said decision reference value with an evaluation value based on the collected data to detect data affected by the subject motion.
In the MR imaging method of the ninth aspect, since the decision reference value is calculated based on the data being collected, a decision reference value adapted to the actual imaging condition can be determined.
In accordance with a tenth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said evaluation value is the sum or average of absolute values of data for one line or a few lines.
In the MR imaging method of the tenth aspect, the fact that the sum or average of absolute values of data for a line with the subject motion is smaller than the sum or average of absolute values of data for a line without the subject motion is utilized to detect data affected by the subject motion.
In accordance with an eleventh aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said evaluation value is the variance of absolute values of data for one line or a few lines.
In the MR imaging method of the eleventh aspect, the fact that the variance of absolute values of data for a line with the subject motion is smaller than the variance of absolute values of data for a line without the subject motion is utilized to detect data affected by the subject motion.
In accordance with a twelfth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said evaluation value is the variation of the sum or average of absolute values of data for one line or a few lines.
In the MR imaging method of the twelfth aspect, the fact that the difference between the sum or average of absolute values of data for a line with the subject motion and the sum or average of absolute values of data for a line without the subject motion is larger than the difference between the sums or averages of absolute values of data for lines without the subject motion is utilized to detect data affected by the subject motion.
In accordance with a thirteenth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said evaluation value is the variation of the variance of absolute values of data for one line or a few lines.
In the MR imaging method of the thirteenth aspect, the fact that the difference between the variance of absolute values of data for a line with the subject motion and the variance of absolute values of data for a line without the subject motion is larger than the difference between the variances of absolute values of data for lines without the subject motion is utilized to detect data affected by the subject motion.
In accordance with a fourteenth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said data is raw data before FFT (fast Fourier transformation) processing.
In the MR imaging method of the fourteenth aspect, since raw data before FFT processing is used to detect data affected by the subject motion, the necessity of re-imaging can be quickly determined.
In accordance with a fifteenth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that said data is raw data subjected to FFT processing.
In the MR imaging method of the fifteenth aspect, since FFT-processed data that corresponds to an actual spatial position is used to detect data affected by the subject motion, alternatives such as use of data in the center of the line become available.
In accordance with a sixteenth aspect, the present invention provides an MRI apparatus characterized in comprising: RF (radio frequency) pulse transmitting means for transmitting an RF pulse; gradient magnetic field applying means for applying a gradient magnetic field; data collecting means for collecting data; pulse sequence control means for executing a pulse sequence according to an LSDI technique in which the step of selectively exciting a linear region and applying an MPG pulse to collect data is repeated; and re-imaging control means for detecting data affected by the subject motion, and re-imaging a region corresponding to said data.
In the MRI apparatus of the sixteenth aspect, the MR imaging method of the first aspect can be suitably implemented.
In accordance with a seventeenth aspect, the present invention provides an MRI apparatus characterized in comprising: RF pulse transmitting means for transmitting an RF pulse; gradient magnetic field applying means for applying a gradient magnetic field; data collecting means for collecting data; pulse sequence control means for executing a pulse sequence according to an LSDI technique in which the step of selectively exciting a linear region and applying an MPG pulse to collect data is repeated; and data interpolating means for detecting data affected by the subject motion, and employing interpolated data in place of said data.
In the MRI apparatus of the seventeenth aspect, the MR imaging method of the fourth aspect can be suitably implemented.
According to the MR imaging method and MRI apparatus of the present invention, an image of high quality can be produced with a reduced number of missing lines by re-imaging a region corresponding to data affected by the subject motion, thus providing ease of diagnosis. Moreover, since interpolated data is employed in place of data affected by the subject motion, the number of missing lines can be reduced without lengthening the imaging time.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.